An implantable cardiac pacer can have various pacing modes as well as various output parameters such as rate, output level (current or voltage, pulse duration or both), sensitivity, refractory period, etc. In some cardiac pacers both the mode, e.g. R-wave inhibited VVI, as well as the various output parameters, are preset during production, whereas in other cardiac pacers either mode or output parameters or both can be altered by external control or programming. Such output parameters and/or pacing mode changes are usually accomplished by the attending physician, usually during an office visit. Therefore, such cardiac pacers may not be responsive to the physiological requirements of the patients. Such requirements fluctuate often during a 24 hour period, certainly more frequently than the interval between patient's visits to the physician. Thus the patient must suffer less than optimum heart pacing.
Increased emphasis is being placed on the use of physiological parameters to control the output parameters, and particularly the rate of stimulation, of implantable cardiac pacers. Such physiological parameters can include activity of the patient (Dahl, U.S. Pat. No. 4,140,132), sensed ionic changes (Wirtzfeld, U.S. Pat. No. 4,202,339) and the stimulus-repolarization interval of the T-wave, (Richards, U.S. Pat. No. 4,228,803). The detection of such changes is utilized either to increase or to decrease the rate of stimulation.
Measurement of physical activity or of the ionic level in the blood does not appear to measure the effectiveness of the pulse emitted from the cardiac pacer initiating myocardial contraction, nor do they permit analysis of the evoked potential, which is generated by the contracting heart. In addition, these do not consider the sympathetic and parasympathetic contributions to cardiac activity. However, measurements of the stimulus to T-wave intervals or repolarization potential does give an indication of effectiveness of stimulation. Absence of repolarization would indicate lack of stimulation; i.e. the heart failed to contract. However, the use of this repolarization to regulate output level (output current or voltage, output pulse duration, or both) could cause delay and irregularity in myocardial contraction or heart rate which the patient may feel.
Detection of the depolarization potential which must, by necessity, precede repolarization can give more immediate information on the presence or absence of effective stimulation within a sufficiently short period of time to permit the resulting intervals between ventricular contractions to be made essentially equal by appropriate stimulating pulse generation.
In order to utilize the presence or absence of the depolarization potential or R-wave, to decrease (or increase) the output level of the cardiac pacer, one must detect the R-wave. When the same electrodes are used to generate pulses to stimulate muscle contraction and to record the resulting depolarization, the detected depolarization is markedly reduced, because it is masked or buried in the exponential decay of the polarization charge on the electrode resulting from the stimulus itself.
In Herscovici, U.S. Pat. No. 4,543,956, issued Oct. 1, 1985, there is described a means for dissipating the polarization charge on the stimulating electrode. Stimulation and sensing of the resulting R-wave is accomplished by the same electrode. The polarization charge is dissipated by the use of a biphasic pulse in which the first phase is of shorter duration and greater amplitude, stimulating the myocardium. The second phase is of the opposite polarity, of lower amplitude and of longer duration than the first. When the polarization charge equals zero i.e. when no net charge is delivered, the second phase of the biphasic pulse drops to zero, and the evoked potential sensing amplifier is triggered into the "on" state.
The present invention pertains to a cardiac pacing system which preferably combines the unipolar and bipolar configuration. It typically uses a bipolar lead which may be placed in the atrium or the ventricle, or a pair of bipolar leads, one in the ventricle and one in the atrium for dual chamber pacing. The invention further pertains to a method of discrimination of the cardiac evoked potential from post stimulus electrode polarization. When the evoked potential is detected, its amplitude and/or configuration can be utilized to regulate automatically, with minimal external control, the implantable cardiac pacer's output characteristics. Thus the pacer itself can regulate the heartbeat in a more naturally physiological manner.